n-butanol is a colorless, neutral liquid of medium volatility with restricted miscibility (about 7-8%) in water, but freely miscible with all common solvent such as glycols, ketones, alcohol, aldehydes, ethers, and aromatic and aliphatic hydrocarbons. n-butanol is used i) to make other chemicals, ii) as a solvent and iii) as an ingredient in formulated products such as cosmetics. The major uses of n-butanol as a feed-stock are in the synthesis of acrylate/methacrylate esters, glycol ethers, n-butyl acetate, amino resins and n-butylamines. Particularly n-butyl acetate is commonly used as solvent in lacquer industry and as flavour in food, beverage, cosmetic and pharmaceutical industries. Currently, more than 3.5 millions tons of n-butanol are consumed annually in the world and it has the potential to capture a significant share of the biofuels market. Thus, an interest is growing for the low cost production of n-butanol in large quantities.
n-butanol can be produced as an acetone/n-butanol/ethanol (ABE) mixture by the fermentation of carbohydrate by solventogenic Clostridia. Another n-butanol production process has been described in the patent application WO2008/052596. It discloses a method for the conversion of glucose to n-butanol by a recombinant Clostridium acetobutylicum strains modified to eliminate the butyrate pathway, the acetone pathway and the lactate pathway.
The inventors observed that in the strain inactivated for butyrate, lactate and acetone synthesis pathways, the production of two-carbon compounds (ethanol and/or acetate) is increased and thus n-butanol yields are not significantly enhanced.
The production of these two-carbon compounds suggests a regulation of expression of genes involved in the four-carbon compounds pathway. The genes involved in the four-carbon compounds pathway are located downstream of thl, a gene coding for thiolase, and are organized in an operon comprising the gene crt coding for crotonase, the gene bcd coding for butyryl-CoA dehydrogenase, the genes etfAB coding for an electron transfer protein and the gene hbd coding for beta-hydroxybutyryl-CoA dehydrogenase. A transcriptional repressor is encoded downstream of the crt-bcd-etfAB-hbd operon. This repressor is encoded by a gene (CAC2713) called rex for “redox-sensing transcriptional repressor”, and is homologous to the gene rex in Bacillus subtilis (Gyan et al., 2006). In this study, it has been shown that the yjlC-ndh operon (coding for a NADH dehydrogenase) is negatively regulated by Rex, depending on the NADH/NAD+ ratio. In the presence of high concentrations of NAD, the repressor Rex binds to the Rex box located not far from the promoter, represses the yjlC-ndh operon and also more largely the NADH consumption pathways. NAD+ boosts the binding activity of Rex whilst NADH seems to have a negligible effect or a partial negative effect on DNA-binding activity.
The problem to be solved by the present invention is to improve the n-butanol production by a modified microorganism. The inventors have found that the deletion of the rex gene leads to an improved production of n-butanol by reducing the production of two-carbon compounds.